Silicon carbide is a ceramic material valued mainly for its high resistance to thermal stress and shock and its exceptional corrosion resistance in high temperature oxidizing environments. It has also found extensive use in the abrasives industry because of its hardness and wear resistance.
In general, silicon carbide exists in both an alpha and a beta form. The alpha phase is characterized as hexagonal, but exhibits many modifications or polytypes based upon stacking sequences in the layered structure. The beta phase, in contrast, is cubic. In both of these structures every atom is tetrahedrally surrounded by four atoms of the other species, forming strong near-covalent bonds. Alpha silicon carbide is assumed to be the stable high temperature phase, and cubic beta silicon carbide transforms slowly to the alpha phase above about 1650.degree. C. Various processes produce predominantly one or the other of these silicon carbide morphologies.
A number of methods of manufacturing silicon carbide have been developed. The most widely used, particularly in large-scale manufacturing, is the so-called Acheson process, in which mixtures of silica and carbon, along with a small amount of sawdust and common salt, are heated in large trough-type electric furnaces. A centrally mounted core of graphite and coke through which a large current can pass serves as a heater element. Maximum temperatures reached in this process approach 2700.degree. C.
Many other methods of manufacturing silicon carbide are disclosed in the literature, for example, in M. Yamamoto's survey article, "Present Situation of SiC Powder," Ceramics, Vol. 22, No. 1, p. 46 (1987). These methods include, for example: (1) the carbothermal reduction of silica and carbon in an inert atmosphere in a vertical furnace; (2) a direct reaction of silicon powder and fine carbon powder at around 1400.degree. C. in an inert atmosphere; (3) a sol-gel silica/carbon reduction process; and (4) a two-stage synthetic silica/carbon reduction process, which is carried out as a gas phase reaction. The two-stage synthetic silica/carbon reduction process involves synthesis of a homogeneous, high-purity mixture of silica and carbon by a gas phase reaction, followed by synthesis of beta-type silicon carbide by a solid state reaction. This method is described as producing spherical, high-purity products having a narrow particle size distribution without aftertreatments.
The methods involving carbothermal reduction of silica at high temperatures are based on a reaction approximating the following stoichiometric equation: EQU SiO.sub.2 +3C.fwdarw.SiC+2CO (gas) (1)
However, it is well-known that the actual reaction mechanism proceeds through the synthesis and subsequent reaction of gaseous silicon monoxide according to the following sequence: EQU SiO.sub.2 +C.fwdarw.SiO (gas)+CO (gas) (2) EQU SiO (gas)+2C.fwdarw.SiC+CO (gas) (3)
A part of the SiC may be formed through a side reaction represented by EQU 2SiO (gas).fwdarw.SiO.sub.2 +Si (4) EQU Si+C.fwdarw.SiC (5)
In both eases, silicon monoxide has an important role in the production of silicon carbide. However, a number of problems must first be overcome to produce silicon carbide powder having desirable properties via the above chemistry.
One problem is that, at reaction temperatures above about 1150.degree. C., silicon monoxide is synthesized according to equation (2) above. The rate of synthesis becomes rapid above about 1600.degree. C. This silicon monoxide tends to condense at cool surfaces near the inlet. Thus, any continuous process must overcome silicon monoxide condensation problems associated with the continuous flow of a silica-containing feed precursor into a hot reaction vessel maintained at a reaction temperature above the generation temperature of the silicon monoxide.
Another problem is that, in addition to the silicon monoxide generation noted above, carbon monoxide is also generated in the reaction sequence of equations (2) and (3) above. Removing the carbon monoxide helps to promote the reaction. However, the gaseous silicon monoxide formed together with the carbon monoxide has a high vapor pressure and tends to be swept away and lost from the reaction chamber unless reacted with carbon. Silicon monoxide loss results in a lowered silicon carbide yield.
One way to reduce the loss of the silicon monoxide is contemplated in U.S. Pat. No. 4,292,276 to Enomoto et al. This patent describes an apparatus for a process in which large excesses of carbon are employed in order to capture the gaseous silicon monoxide gas before it can escape. These excesses are in the carbon/silica molar ratio range of 3.2 to 5. Unfortunately, this method results in a reaction product which contains a large excess of carbon and relatively little silicon carbide.
Another invention addressing the problem of silicon monoxide loss is disclosed in U.S. Pat. No. 4,368,181 (Suzuki et al.), which describes a two-step process wherein silicon monoxide gas is first synthesized according to reaction equation (2) above and then captured via condensation at low temperatures. In the second step the captured condensed silicon monoxide is pulverized with carbon and silica and further reacted to form silicon carbide.
Another problem encountered in any continuous silicon carbide producing process is the continuous discharge of condensing fluids through the outlet end of the furnace. It is difficult to prevent the condensation of any remaining gaseous silicon monoxide along the inside walls of the cooling zone in those furnace designs having a specified cooling area. Unless all of the silicon monoxide reacts completely to silicon carbide within the reaction chamber, some will exit the reaction chamber with the carbon monoxide. The result is that silicon monoxide will condense and deposit within the inlet of the cooling zone, again often causing plugging problems and preventing continuous operation.
Still another problem often encountered in preparing silicon carbide, particularly beta silicon carbide, by currently known methods is that uniform, pure product is difficult to achieve. Purity and uniformity of size and morphology have been found to be desirable for powders used to produce many engineered ceramics products because these properties can help to reduce the incidence of failure due to the presence of small cracks or voids that result from incomplete packing of the precursor powders. It has been suggested by E. A. Barringer and H. K. Bowen, in "Formation, Packing and Sintering of Monodispersed TiO.sub.2 Powders," J. Amer. Ceram. Soc. 65, C-199 (1982), that, in general, an `ideal` ceramic powder for producing a high quality part should be of high purity and contain particles which are spherical, nonagglomerated and of a relatively uniform particle size ranging from about 0.1 to about 1.0 micrometer in diameter. The uniform and fine powders often densify at lower temperatures, thus representing cost savings in the long run and, because of their optimized packing capability, often producing significantly stronger and thus more reliable parts. In silicon carbide production, however, it has proven difficult to achieve the desired particle size and uniformity.
For example, when silicon carbide powder is manufactured commercially by the so-called "Acheson process" described above, the result is commonly an extremely nonuniform product. This is because the heating rate is slow and the mass of reactants does not heat evenly. Extensive size reduction, classification, and acid leaching of the product are necessary in order to prepare powders suitable for part fabrication. The size reduction, done by milling of some sort, such as attrition milling, is time-consuming and allows introduction of impurities.
Because of these problems, researchers have sought methods of directly producing silicon carbide having the desired particle size range and uniformity. One effective method involves the direct synthesis of these powders from laser-heated gases. For example, R. A. Marra and J. S. Haggerty, in their article, "Synthesis and Characteristics of Ceramic Powders Made from Laser-Heated Gases," Ceram. Eng. Sci. Proc. 3, 31 (1982), describe the preparation of silicon carbide powder by driving exothermic reactions involving SiH.sub.4. The result is equiaxed, monodispersed powders with particle sizes in the range of from 0.01 to 0.1 micrometer.
Powders having a desirable size and purity level have also been successfully synthesized from radio frequency plasma-heated gases. See, e.g., U.S. Pat. No. 4,266,977 to Steiger. In another gas phase type synthesis process, U.S. Pat. No. 3,346,338 to Latham, Jr., discloses the continuous production of finely-divided silicon carbide by passing a vapor of each reactant into one end of a furnace reaction zone and then recovering from the other end of the reaction zone a finely-divided carbide product.
In general, the laser- or plasma-heating of reactant gases is characterized by almost instantaneous heating rates of reactants, short reaction times (fractions of a second), minimal exposure to high temperatures, and almost instantaneous product cooling rates. The net result of the nearly instantaneous and uniform heating is submicrometer, uniformly sized ceramic particles. However, while gas phase synthesized powders possess many desirable qualities, the powders are relatively expensive to produce because of the inherent low production rate and high cost of the required equipment and gaseous raw materials. Thus, the gas phase routes, while academically intriguing, may encounter serious limitations to commercial use.
Efforts to directly produce uniform, fine powders by less expensive, more commercially practicable means have also included various furnace modifications. In general, these means involve passing solid reactants through a heated, relatively restricted space, containing inert or reaction-compatible gases, at a rate determined by the desired reaction and the need to avoid decomposition of the desired product. For example, vertical tubular reactors having a general configuration suitable for this are described in a number of patents to Matovich (e.g., U.S. Pat. Nos. 3,933,434; 4,042,334; 4,044,117; 4,056,602; 4,057,396; 4,095,974; 4,199,545; and 4,234,543). These reactors have an inlet end, a reaction chamber, and an outlet end, and the reaction chamber is defined as the interior of an envelope of inert fluid which protects the inside tube wall from both the reactants and the products of reaction. Various processes utilizing these reactors are described in these patents, and silicon carbide is suggested as a possible product in, for example, U.S. Pat. No. 3,933,434. However, the properties of the silicon carbide are neither described nor postulated.
U.S. Pat. Nos. 4,162,167, 4,292,276 and 4,529,575 to Enomoto also disclose an apparatus suitable for producing silicon carbide. In this case the product consists mainly of beta-type crystals. The Enomoto apparatus is a vertical-type reaction vessel having an inlet for a starting material, a reaction zone and a closeable outlet for a product in this order. The closeable outlet allows extended reaction times, on the order of hours. When the process is carried out using excess carbon, the result is a product having an average particle size of greater than one micrometer. The particle size distribution is unspecified.
G. C. Wei, in "Beta SiC Powders Produced by Carbothermic Reduction of Silica in a High-Temperature Rotary Furnace," Communications of the American Ceramic Society, July 1983, describes another process for producing silicon carbide. The product has a spherical diameter, based on a Brunauer-Emmett-Teller (BET) surface area range, of from 0.3 to 9 micrometers. The particle size distribution is again unspecified.
Finally, U.S. Pat. No. 4,368,181 to Suzuki et al describes a method of producing inexpensive beta-type silicon carbide by reacting silica having a particle size of less than 150 micrometers and carbon having a particle size of less than 60 micrometers at a temperature below 1650.degree. C. The silicon monoxide formed during the reaction is then contacted again with unreacted carbon to increase the yield. The resulting product is described as consisting mainly of particles having a size of 0.04 to 0.08 micrometers, in the case of a reaction temperature of about 1450.degree. C., and 0.1 to 0.3 micrometer, in the case of a reaction temperature of about 1600.degree. C. Particle size distribution is unspecified.
Thus, it would be desirable in the art to develop a continuous method of producing uniform, fine silicon carbide powders, which reduces or avoids the problems described above and which results in a uniform, high-purity product of a desirable size range and particle size distribution.